The achilleas heel of the Bissell Proheat is the heater, it has a bad habit of becoming clogged up with detergent, take it to pieces and thoroughly clean it out including the input and exit valves, there are a lot of videos on YouTube showing you how to do this.
If you keep getting this problem, it maybe easier to bypass the heater altogether with a new piece of tubing.
Whilst your about it check the nozzles where the water ejects onto your carpet, these also become clogged up.
Delshan
August 2020